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LECTURE ON AM/FM TRANSMITTER

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LECTURE ON AM/FM TRANSMITTER

Block diagram of communication system

TRANSMITTER

It takes the information to be communicated in electrical form and convert it into an electronic signal compatible with the communication medium .

Information

in Electrical form

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TRANSMITTER

In this block diagram of communication system, the upper section is called the transmitting section.

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TRANSMITTER

The main parts of transmitter are explained as follows :

Microphone : It converts sounds into electrical signals in wires. It is the opposite of a loudspeaker.

Modulator : The audio signal is modulated into the radio frequency carrier in this modulator stage.

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TRANSMITTER

Frequency generator : The frequency generation stage will decide the frequency on which the transmitter will operate.

RF power amplifier : The power amplification of the radio signal is carried out in the final stage. It makes the signal stronger so that it can be transmitted through the channel over long distances.

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TRANSMITTER

An antenna is a transducer which converts electrical signals into electromagnetic waves.

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Modulator

RF oscillator

Power amplifier

BASIC BLOCKS OF TRANSMITTER

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Modulation

Carrier generation

Amplification (Power)

It is an electronic unit which accepts the information signal to be transmitted and converts it into an RF signal capable of being transmitted over long distances .

BASIC FUNCTIONS OF TRANSMITTER

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Every transmitter has three basic functions as follows:

The transmitter must generate a signal of correct frequency at a desired point in the spectrum.

Secondly it must provide some form of modulation to modulate the carrier.

Third it must provide sufficient power amplification in order to carry the modulated signal to a long distance.

BASIC FUNCTIONS OF TRANSMITTER

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CLASSIFICATION OF RADIO TRANSMITTERS

According to the type of modulation used.

According to service involved.

According to the frequency range involved.

According to the power used.

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Low frequency (LF) transmitters (30 KHZ- 300KHZ)

Medium frequency (MF) transmitters ( 300 KHZ-3 MHZ)

High frequency (HF) transmitters (3 MHZ- 30MHZ)

CLASSIFICATION BASED ON TRANSMITTED FREQUENCY

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CLASSIFICATION BASED ON TRANSMITTED FREQUENCY

Very high frequency (VHF) transmitters (30MHZ-300 MHZ)

Ultra high frequency (UHF) transmitters (300 MHZ- 3GHZ)

Microwave transmitters (>3GHZ)

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CLASSIFICATION BASED ON TYPE OF SERVICE INVOLVED

Radio broadcast transmitters.

Radio telephony transmitters.

Radio telegraph transmitters.

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CLASSIFICATION BASED ON TYPE OF SERVICE INVOLVED

Television transmitters.

Radar transmitters.

Navigational transmitters.

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CW Transmitters

AM Transmitters

FM Transmitters

SSB Transmitters

CLASSIFICATION BASED ON TYPES OF MODULATION

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The CW Transmitter is the simplest type of transmitter.

It is a simple crystal oscillator circuit.

This oscillator generates a carrier signal of the desired frequency.

CONTINIOUS WAVE (CW) TRANSMITTERS

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Information to be transmitted is expressed in a special form of code using dots and dashes to represent letters of the alphabet and numbers.

The information transmitted in this way is called as continuous wave (CW) transmission.

CONTINIOUS WAVE (CW) TRANSMITTERS

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CONTINIOUS WAVE (CW) TRANSMITTERS

The key is a simple hand operated switch connected in emitter of the transistor.

By closing the key we can turn on the crystal oscillator on and by opening the key the oscillator is turned off.

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When the key is closed, the oscillator produces a sinusoidal signal at a frequency equal to the crystal frequency, whereas with the key open, the output of oscillator is zero.

The key is opened and closed in order to produce zero output and dots or dashes.

CONTINIOUS WAVE (CW) TRANSMITTERS

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CONTINIOUS WAVE (CW) TRANSMITTERS

Dots correspond to the short duration output whereas a dash corresponds to a long duration output.

The required messages can be transmitted using different combinations of dots and dashes for different alphabets and letters.

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Simple to construct

Compact and portable

Can be operated on batteries

ADVANTAGES OF CW TRANSMITTER

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A skilled operator is required to convert the message to be sent into a coded form of dots and dashes.

Long distance communication is not possible.

Voice or picture can not be sent.

DISADVANTAGES

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Amplitude modulation technique is used in AM transmitters, here the amplitude of carrier is varied in proportion with the amplitude of the modulating signal, keeping its frequency and phase constant.

Used in radio & TV broadcasting.

AM TRANSMITTER

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AM TRANSMITTER

In AM Transmitter, AM signal is transmitted by a transmitter. The information is contained in its amplitude variation.

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Low Level modulation transmitters.

High Level modulation transmitters.

TYPES OF AM TRANSMITTERS

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The generation of AM wave takes place at a low power level.

The generated AM signal is then amplified using a chain of linear amplifier ( A , AB or B).

Low Level

Modulator

Power Amplifier

(Linear)

RF Carrier Oscillator

LOW LEVEL MODULATION TRANSMITTERS

Signal

Source

(Modulating

Signal)

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STABILISEDRF OSCILLATOR

LOW LEVEL MODULATION TRANSMITTERS

Antenna

Class A Buffer amplifier

Power Amplifiers

Linear Amplifiers

Modulator

Audio processing & filtering

Class A AF amplifier

AF modulating signal

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LOW LEVEL MODULATION TRANSMITTERS

The RF oscillator produces the carrier signal. The RF oscillator is stabilized in order to maintain the frequency deviation within the prescribed limit. The carrier frequency is equal to the transmitter frequency.

Usually the transmitter operates on assigned frequencies or channels. Crystal provides the best way to obtain the described frequency with good stability.

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LOW LEVEL MODULATION TRANSMITTERS

We cannot use the LC oscillator because they have low frequency stability.

The carrier signal from the crystal oscillator is applied to the modulator with a modulating signal. At the output of the modulator we get the AM wave.

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LOW LEVEL MODULATION TRANSMITTERS

The modulating signal is obtained from a source such as a microphone and applied to a buffer processing unit.

The buffer is a class A amplifier which isolates the AF source from the rest of high power circuit and amplifies it to an adequate level.

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LOW LEVEL MODULATION TRANSMITTERS

The amplified modulating signal is applied to the modulator along with the carrier. At the output of the modulator we get the AM wave.

The AM signal is then amplified using a chain of linear amplifier to raise the power level.

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LOW LEVEL MODULATION TRANSMITTERS

The linear amplifier can be class A, AB or B type amplifiers. The linear amplifier are used in order to avoid the wave form distortion in AM wave.

The amplitude modulated signal is then transmitted using transmitted antenna.

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LOW LEVEL MODULATION TRANSMITTERS

The transistorized modulator circuits can be used for low level modulator due to the low power which is to be handled.

The low level transmitter does not require a large AF modulator power so its design is simplified.

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LOW LEVEL MODULATION TRANSMITTERS

Overall efficiency is much lower compared to high level modulation . This reduce to the use of less efficient linear amplifiers.

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The AF modulating signal is passed through an audio processing unit before applying it to the modulator.

This block carries out some form of “speech processing” in the form of filtering and amplitude control.

The weak signals amplified automatically with a higher gain and strong signals are amplified with smaller gain. This will bring all the signals to a sufficient level.

AUDIO PROCESSING

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The generation of AM wave takes place at high power levels.

Wide Band

Power Amplifier

High Level Modulator

RF Carrier Oscillator

Narrow Band Power Amplifier

HIGH LEVEL MODULATION TRANSMITTERS

Signal

Source

(Modulating

Signal)

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HIGH LEVEL MODULATION TRANSMITTERS

Highly efficient class C amplifier are used in high level modulation.

Efficiency is more than low level modulation.

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Stabilized RF crystal oscillator

HIGH LEVEL MODULATION TRANSMITTERS

Antenna

Class A RF amplifier

Class C RF power amplifier

High Level Modulator

Audio processing & filtering

Class A AF amplifier

Class B AF power amplifier

AF modulating signal

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Many of the AM transmitters use the high level modulation technique.

The crystal oscillator produces the required carrier signal. The class A amplifier following the oscillator acts as a buffer which isolates the oscillator from the high power circuit.

HIGH LEVEL MODULATION TRANSMITTERS

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HIGH LEVEL MODULATION TRANSMITTERS

The output of this class A amplifier is applied to a class C power amplifier. It raises the power level of the carrier to an intermediately high value.

The AF modulating signal is applied to the audio processing unit which processes this signal as discussed in the previous section.

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Power level :

Modulation is carried out at low power level.

Amplifier stages:

Need lesser amplifier stages.

Modulation is carried out at high power level.

Need more amplifier stages.

COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL MODULATION

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Power efficiency :

After modulation linear amplifiers can only be used. This gives lower power efficiency.

Power losses :

Power losses in amplifiers is higher, the cooling problem is severe.

Non linear amplifiers can also be used. This leads to higher power efficiency.

Power losses is less, the cooling problem is not severe.

COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL MODULATION

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Applications :

Used as higher power broadcast transmitters.

Used in TV transmitters.

COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL MODULATION

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Frequency modulation technique is used.

In FM frequency of the carrier is varied in proportion with the amplitude of the modulating signal keeping its amplitude constant.

FM TRANSMITTERS

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FM TRANSMITTERS

Used in radio, TV sound broadcasting & police wireless transmission.

In FM transmitter the FM signal is transmitted by a transmitter. The information is contained in its frequency variation.

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FM TRANSMITTERS

The FCC has assigned a band of 20 MHz to the commercial FM broad cast service.

This band extends from 88 MHz to 108 MHz.

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This 20 MHz band is divided in 100 channels, each having a bandwidth of 200 KHz.

For providing high quality reliable music the maximum frequency deviation allowed is 75 KHz, with a maximum modulating signal frequency of 15 KHz.

FM TRANSMITTERS

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Direct Methods

Indirect Methods

METHODS OF FM GENERATION

Methods of FM Generation

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In direct FM generation the frequency of the carrier is changed directly in proportion with the modulating signal amplitude.

Types of Direct FM

Reactance modulator

Varactor diode modulator

DIRECT FM

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DIRECT FM

These methods use a varactor diode or a reactance transistor for presenting a variable reactance across the frequency determining circuit of an oscillator.

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DIRECT FM

Modulating

Signal

Oscillator Tank

Circuit

Variable reactance device

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DIRECT FM

When the variable reactance of the device varies with the modulating signal the oscillator generator the corresponding FM signal.

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DIRECT MODULATORS

Few other direct modulators are:

Varactor diode modulator.

Reactance modulator.

V.C.O. modulator.

Stabilized reactance modulator.

Cross by direct FM transmitters.

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REACTANCE MODULATOR

In the reactance modulator, a transistor or FET is operated as a variable reactance (inductive or capacitive) device.

This device is connected across the tuned circuit of an oscillator.

As the instantaneous value of modulating voltage changes, the reactance offered by the transistor or FET will change proportionally.

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Here the voltage applied across the varactor diode varies in proportion with the modulating voltage.

This will vary the junction capacitance of the varactor diode. The varactor diode appears in parallel with the oscillator tuned circuit. Hence the oscillator frequency will change with change in varactor diode capacitance and FM wave is produced.

VARACTOR DIODE

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The main advantage of direct FM generation is the simplicity of the modulators and their low cost.

ADVANTAGES OF DIRECT FM MODULATION

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In the direct method of FM generation we have to use the LC oscillator. The LC oscillator frequency is not stable.

Therefore its not possible to use such oscillator for communication or broadcast purpose.

DISADVANTAGES OF DIRECT METHOD

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DIRECT FM

Therefore we have to use a scheme in which we can use the crystal oscillator to control the carrier frequency.

Therefore we have to use the automatic frequency control system.

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Here FM is obtained through phase modulation.

A Crystal oscillator is used and hence the frequency stability is very high.

A narrow band PM signal is generated via DSB-SC signal and a 90° phase shifted sub carrier signal from a crystal oscillator.

INDIRECT METHOD ( ARMSTRONG METHOD)

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INDIRECT METHOD ( ARMSTRONG METHOD)

The integration of the modulating signal makes the NBPM signal to NBFM signal.

This NBFM signal is applied to a harmonic generator (frequency multiplier) which will increase both the carrier frequency and the maximum deviation to the required.